U.S. patent number 8,704,200 [Application Number 13/441,639] was granted by the patent office on 2014-04-22 for laser produced plasma euv light source.
This patent grant is currently assigned to Cymer, LLC. The grantee listed for this patent is Alexander N. Bykanov, Igor V. Fomenkov. Invention is credited to Alexander N. Bykanov, Igor V. Fomenkov.
United States Patent |
8,704,200 |
Bykanov , et al. |
April 22, 2014 |
Laser produced plasma EUV light source
Abstract
An EUV light source is disclosed which may comprise a plurality
of targets, e.g., tin droplets, and a system generating pre-pulses
and main-pulses with the pre-pulses for irradiating targets to
produce expanded targets. The system may further comprise a
continuously pumped laser device generating the main pulses with
the main pulses for irradiating expanded targets to produce a burst
of EUV light pulses. The system may also have a controller varying
at least one pre-pulse parameter during the burst of EUV light
pulses. In addition, the EUV light source may also include an
instrument measuring an intensity of at least one EUV light pulse
within a burst of EUV light pulses and providing a feedback signal
to the controller to vary at least one pre-pulse parameter during
the burst of EUV light pulses to produce a burst of EUV pulses
having a pre-selected dose.
Inventors: |
Bykanov; Alexander N. (San
Diego, CA), Fomenkov; Igor V. (San Diego, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Bykanov; Alexander N.
Fomenkov; Igor V. |
San Diego
San Diego |
CA
CA |
US
US |
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Assignee: |
Cymer, LLC (San Diego,
CA)
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Family
ID: |
39541508 |
Appl.
No.: |
13/441,639 |
Filed: |
April 6, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120193547 A1 |
Aug 2, 2012 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12928313 |
Dec 7, 2010 |
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11644153 |
Apr 19, 2011 |
7928416 |
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Current U.S.
Class: |
250/504R; 372/55;
372/25 |
Current CPC
Class: |
H05G
2/003 (20130101); G03F 7/70033 (20130101); H05G
2/005 (20130101); H05G 2/008 (20130101) |
Current International
Class: |
H05G
2/00 (20060101) |
Field of
Search: |
;250/504R
;372/25,55 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10149654 |
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Apr 2003 |
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DE |
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1492394 |
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Dec 2004 |
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EP |
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2003-008124 |
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Jan 2003 |
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JP |
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2003-272892 |
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Sep 2003 |
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JP |
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2005-017274 |
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Jan 2005 |
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JP |
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Other References
US. Appl. No. 60/775,442, filed Feb. 21, 2006, Bowering, et al.
cited by applicant .
PCT International Search Report dated Nov. 19, 2007 from
International Application No. PCT US2006/06947, filed Feb. 24, 2006
PCT (2 pgs). cited by applicant .
Written Opinion from PCT International Search Report dated Nov. 19,
2007 from International Application No. PCT US2006/06947, filed
Feb. 24, 2006 (3 pgs). cited by applicant .
European Search Report dated May 10, 2010 from European Patent
Application No. 07862537.3, filed on Dec. 4, 2007 (10 pgs). cited
by applicant .
Japanese Official Action dated Jun. 25, 2012, filed in counterpart
Japanese Patent Application No. 2009-542786, filed on Dec. 4, 2009
(3 pages). cited by applicant .
Taiwan Official Letter dated May 11, 2012, in counterpart Taiwan
Patent Application No. 96144942, filed Nov. 27, 2007 (1 pg). cited
by applicant.
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Primary Examiner: Maskell; Michael
Attorney, Agent or Firm: Cymer, LLC
Parent Case Text
This application is a Continuation of U.S. patent application Ser.
No. 12/928,313, filed on Dec. 7, 2010, and published on Apr. 7,
2011 as US 2011-0079736-A1, entitled LASER PRODUCED PLASMA EUV
LIGHT SOURCE, which is a Divisional of U.S. patent application Ser.
No. 11/644,153 filed on Dec. 22, 2006, and issued on Apr. 19, 2011,
as U.S. Pat. No. 7,928,416, entitled LASER PRODUCED PLASMA EUV
LIGHT SOURCE, the contents of which are incorporated herein by
reference.
The present application is related to U.S. patent application Ser.
No. 11/358,988, filed on Feb. 21, 2006, and published on Nov. 16,
2006, as U.S. 2006-0255298A1, entitled LASER PRODUCED PLASMA EUV
LIGHT SOURCE WITH PRE-PULSE, U.S. patent application Ser. No.
11/067,124 filed on Feb. 25, 2005, now U.S. Pat. No. 7,405,416,
issued on Jul. 29, 2008, entitled METHOD AND APPARATUS FOR EUV
PLASMA SOURCE TARGET DELIVERY, U.S. patent application Ser. No.
11/174,443 filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056,
issued on May 13, 2008, entitled LPP EUV PLASMA SOURCE MATERIAL
TARGET DELIVERY SYSTEM, U.S. patent application Ser. No. 11/358,983
filed on Feb. 21, 2006, now U.S. Pat. No. 7,378,673, issued on May
27, 2008, entitled TARGET MATERIAL DISPENSER FOR EUV LIGHT SOURCE,
U.S. patent application Ser. No. 11/358,992 filed on Feb. 21, 2006,
now U.S. Pat. No. 7,598,509, issued on Oct. 6, 2009, entitled LASER
PRODUCED PLASMA EUV LIGHT SOURCE, U.S. Provisional Patent
Application Ser. No. 60/775,442 filed on Feb. 21, 2006, entitled
EXTREME ULTRAVIOLET LIGHT SOURCE, U.S. patent application Ser. No.
11/174,299 filed on Jun. 29, 2005, now U.S. Pat. No. 7,439,530,
issued on Oct. 21, 2008, and entitled, LPP EUV LIGHT SOURCE DRIVE
LASER SYSTEM, U.S. patent application Ser. No. 11/406,216 filed on
Apr. 17, 2006, now U.S. Pat. No. 7,465,946, issued on Dec. 16,
2008, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, and U.S.
patent application Ser. No. 11/580,414 filed on Oct. 13, 2006, now
U.S. Pat. No. 7,491,954, issued on Feb. 17, 2009, entitled, DRIVE
LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, the entire contents of
each of which are hereby incorporated by reference herein.
Claims
What is claimed is:
1. EUV light source comprising: a target material delivery system
for delivering a plurality of targets; a system generating laser
pulses on an output beam path, the laser pulses for irradiating
targets to produce a burst of EUV light pulses; an instrument
measuring an intensity of at least a portion of one EUV light pulse
within a burst of EUV light pulses and providing a feedback signal
indicative thereof; and a shutter positioned on the output beam
path responsive to the feedback signal to trim a laser pulse, the
trimmed laser pulse generating an EUV light pulse during the burst
of EUV light pulses.
2. A light source as recited in claim 1 wherein the portion of the
EUV pulse measured is selected from the group of portions
consisting of a temporal portion, a spatial portion and a temporal
and spatial portion.
3. A light source as recited in claim 1 wherein the portion of the
EUV pulse measured is a temporal portion and the EUV pulse
generating the measured intensity is produced by the trimmed laser
pulse.
4. A light source as recited in claim 3 wherein said system
comprises an optical oscillator outputting laser pulses with each
output laser pulse having a pulse duration, t, and wherein the
travel time, T, for light traversing an optical path extending from
the optical oscillator output to the target and from the target to
an intensity measurement site is less than the pulse duration
exiting the oscillator, T<t.
5. A light source as recited in claim 4 wherein the system further
comprises at least one optical amplifier receiving laser light from
the optical oscillator along an optical path and the shutter is
positioned on the optical path between the optical amplifier and
the optical oscillator.
6. A light source as recited in claim 1 wherein the EUV pulse
measured to produce the feedback signal is generated by a
corresponding laser pulse and the feedback signal produced is used
to trim a subsequent laser pulse.
7. A light source as recited in claim 1 wherein said shutter
comprises an electro-optic switch.
8. A light source as recited in claim 1 wherein said system
generating laser pulses comprises an optical oscillator and at
least one optical amplifier.
9. A light source as recited in claim 8 wherein the optical
oscillator is a cavity dumped oscillator.
10. A light source as recited in claim 8 wherein the optical
oscillator is a transverse electrical-discharge oscillator having a
pulsed power source.
11. An EUV light source as recited in claim 8 wherein the amplifier
comprises a continuously pumped laser device.
12. An EUV light source as recited in claim 1 wherein said system
generating laser pulses comprises an optical oscillator defining a
primary polarization direction and the shutter comprises an
electro-optic switch having an electro-optic cell and a
polarizer.
13. An EUV light source as recited in claim 1 further comprising a
chamber, the laser pulses irradiating targets in the chamber, and
wherein the intensity measuring instrument is positioned outside
the chamber.
14. An EUV light source as recited in claim 13 wherein the
intensity measuring instrument is positioned in a lithography
scanner coupled to the chamber.
15. An EUV light source as recited in claim 13 wherein the
intensity measuring instrument is positioned at a location
optically downstream of a lithography scanner coupled to the
chamber.
16. An EUV light source comprising: a target material delivery
system for delivering a plurality of targets; a system generating
laser pulses on an output beam path, the laser pulses for
irradiating targets to produce EUV light pulses; a means for
measuring an intensity of at least a portion of one EUV light pulse
and providing a feedback signal indicative thereof; and a laser
pulse trimming means positioned on the output beam path responsive
to the feedback signal to trim a laser pulse.
17. A light source as recited in claim 16 wherein the portion of
the EUV pulse measured is selected from the group of portions
consisting of a temporal portion, a spatial portion and a temporal
and spatial portion.
18. A light source as recited in claim 16 wherein the portion of
the EUV pulse measured is a temporal portion and the EUV pulse
generating the measured intensity is produced by the trimmed laser
pulse.
19. A method of producing EUV light comprising the steps of:
providing a plurality of targets; generating laser pulses on an
output beam path, the laser pulses for irradiating targets to
produce EUV light pulses; measuring an intensity of at least a
portion of one EUV light pulse and providing a feedback signal
indicative thereof; and trimming, using a shutter, a laser pulse on
the output beam path in response to the feedback signal.
20. The method as recited in claim 19 wherein the portion of the
EUV pulse measured in said measuring step is a temporal portion and
the EUV pulse generating the measured intensity is produced by the
trimmed laser pulse.
Description
FIELD
The present disclosure relates to extreme ultraviolet ("EUV") light
sources which provide EUV light from a plasma that is created from
a target material and collected and directed to a focus for
utilization outside of the EUV light source chamber, e.g., for
semiconductor integrated circuit manufacturing photolithography
e.g., at wavelengths of around 50 nm and below.
BACKGROUND
Extreme ultraviolet light, e.g., electromagnetic radiation having
wavelengths of around 50 nm or less (also sometimes referred to as
soft x-rays), and including light at a wavelength of about 13.5 nm,
can be used in photolithography processes to produce extremely
small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily
limited to, converting a material into a plasma state that has an
element, e.g., xenon, lithium or tin, with one or more emission
line in the EUV range. In one such method, often termed laser
produced plasma ("LPP") the required plasma can be produced by
irradiating a target material, such as a droplet, stream or cluster
of material having the required line-emitting element, with a laser
beam.
Heretofore, LPP systems have been disclosed in which each droplet
is irradiated by a separate laser pulse to form a plasma from each
droplet. Also, systems have been disclosed in which each droplet is
sequentially illuminated by more than one light pulses. In some
cases, each droplet may be exposed to a so-called "pre-pulse" and a
so-called "main pulse", however, it is to be appreciated that more
than one pre-pulse may be used and more than one main pulse may be
used and that the functions of the pre-pulse and main pulse may
overlap to some extent. Typically, the pre-pulse(s) may function to
expand the material and thereby increase the amount of material
which interacts with the main pulse and the main-pulse may function
to convert most or all of the material into a plasma and thereby
produce an EUV light emission. However, it is to be appreciated
that the functions of the pre-pulse and main pulse may overlap to
some extent, e.g., the pre-pulse(s) may generate some plasma, etc.
The increased material/pulse interaction may be due a larger
cross-section of material exposed to the pulse, a greater
penetration of the pulse into the material due to the material's
decreased density, or both. Another benefit of pre-pulsing is that
it may expand the target to the size of the focused pulse, allowing
all of the pulse to participate. This may be especially beneficial
if relatively small droplets are used as targets and the
irradiating light cannot be focused to the size of the small
droplet.
In addition to the above described techniques, U.S. Pat. No.
6,855,943 (hereinafter the '943 patent) which issued to Shields on
Feb. 15, 2005 and is entitled "DROPLET TARGET DELIVERY METHOD FOR
HIGH PULSE-RATE LASER-PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE"
discloses a technique in which only some of the droplets in a
droplet stream, e.g., every third droplet, is irradiated to produce
a pulsed EUV light output. As disclosed in the '943 patent, the
nonparticipating droplets (so-called buffer droplets)
advantageously shield the next participating droplet from the
effects of the plasma generated at the irradiation site.
Unfortunately, in some cases, these buffer droplets may reflect
light back into the laser causing self-lasing, which among other
things, can reduce the effectiveness of the laser's gain media in
producing high energy pulses. This may be especially true for high
gain (e.g., G=1000-10,000) infra-red lasers, e.g., CO.sub.2 lasers,
which tend to self-lase rather easily.
A typically photolithography scanner exposes a portion of a moving
wafer to a so-called "burst" of light pulses. In many cases, the
pulse energy varies from pulse-to-pulse within the burst, and the
accumulated energy of the burst, referred to generally as "dose",
is typically prescribed and must be controlled within a relatively
small range. In addition to dose, some lithography operations also
prescribe limits on the pulse-to-pulse energy variation within a
burst. This is sometimes referred to as pulse-to-pulse energy
stability. The lasers that are used in deep ultraviolet
photolithography (DUV), e.g., excimer lasers at a wavelength of,
e.g., 100-300 nm, typically have the ability to substantially vary
pulse energy within a burst of pulses, and in some cases, on a
pulse-to-pulse basis. This can be achieved, for example, by
altering the discharge voltage used to create each pulse. This
flexibility, however, is not available for all types of lasers. In
some laser architectures, a continuously pumped active media may be
used to generate laser pulses. For example, pulses may be generated
in a continuously pumped oscillator using, for example, Q-switch or
cavity dump mode, and then amplified by one or more continuously
pumped power amplifiers. As used herein, the term continuously
pumped laser devices (CW laser devices) refers to a laser device
having a continuously pumped active media, e.g., continuously
pumped oscillator and/or continuously pumped amplifier. Unlike the
pulse--pumped laser devices described above, e.g., excimer
discharge laser devices, the CW laser devices do not have the
ability to quickly vary their discharge voltage, and as a
consequence, are generally incapable of substantially varying their
output pulse energy within a burst of pulses by altering their
discharge voltage.
As indicated above, one technique to produce EUV light involves
irradiating a target material droplet with one or more pre-pulse(s)
followed by a main pulse. In this regard, CO.sub.2 lasers, and in
particular, continuously pumped CO.sub.2 laser devices may present
certain advantages as a drive laser producing "main" pulses in an
LPP process. This may be especially true for certain targets, e.g.,
tin. For example, one advantage may include the ability to produce
a relatively high conversion efficiency e.g., the ratio of output
EUV in-band power to drive laser input power.
Along these lines, U.S. Pat. No. 6,973,164 which issued to Hartlove
et al. on Dec. 6, 2005 and is entitled "LASER-PRODUCED PLASMA EUV
LIGHT SOURCE WITH PRE-PULSE ENHANCEMENT" discloses that a variation
in time delay between pre-pulse and main pulse in a Nd:YAG with
Xenon targets LPP system results in a variation of output EUV
intensity for time delays shorter than 160 ns.
With the above in mind, Applicants disclose a laser produced plasma
EUV light source, and corresponding methods of use.
SUMMARY
In a first aspect of an embodiment of the present application, an
EUV light source may comprise a plurality of targets, e.g., tin
droplets, and a system generating pre-pulses and main-pulses with
the pre-pulses for irradiating targets to produce expanded targets.
The system may further comprise a continuously pumped laser device
generating the main pulses with the main pulses for irradiating
expanded targets to produce a burst of EUV light pulses. The system
may also have a controller varying at least one pre-pulse parameter
during the burst of EUV light pulses. In addition, the EUV light
source may also include an instrument measuring an intensity of at
least a portion of one EUV light pulse within a burst of EUV light
pulses and providing a feedback signal to the controller to vary at
least one pre-pulse parameter during the burst of EUV light
pulses.
For this aspect, the pre-pulse parameter may be a delay time
between a pre-pulse and the corresponding main pulse and/or the
pre-pulse parameter may be the pulse energy of the pre-pulse. In
one embodiment of this aspect, the continuously pumped laser device
may comprise a CO.sub.2 laser device and in a particular embodiment
the pre-pulses may pass through the continuously pumped laser
device.
In another aspect, a method for generating EUV light may comprise
the steps/acts of irradiating at least one target material
droplet(s) with an initial light pulse to create irradiated target
material, exposing irradiated target material to a subsequent light
pulse to generate EUV light, measuring an intensity of the EUV
light generated, using the measurement to calculate an energy
magnitude for a subsequent light pulse, and thereafter irradiating
a target material droplet with a light pulse having the calculated
energy magnitude. In one implementation, the subsequent light pulse
may be generated using a continuously pumped laser device and in a
particular implementation, the continuously pumped laser device may
comprise a CO.sub.2 laser device and the target material may
comprise tin.
For another aspect of an embodiment, an EUV light source may
comprise a plurality of target material droplets traveling along a
path, e.g., a substantially linear path, and a system generating an
initial pulse and a subsequent pulse. The initial pulse is provided
for irradiating a droplet on the path to generate irradiated target
material exiting the path, and the subsequent pulse is focused to a
location for interaction with the irradiated target material. For
this aspect, the location is distanced from the path and the
subsequent pulse is provided for exposing the irradiated target
material to generate EUV light.
In one implementation of this aspect, only every other droplet
traveling along the path is irradiated by a respective pre-pulse.
In some embodiments, the initial pulse and the subsequent pulse may
travel along a common beam path. In particular, the first and
subsequent pulse may be focused using a common optic and the
initial pulse and the subsequent pulse may have differing beam
divergences prior to focusing. The light source may further
comprise one or more saturable absorbers positioned to absorb
photons reflected from the target material. Alternatively, or in
addition thereto, the system may comprise an amplifier amplifying
the subsequent pulse and the light source may further comprise an
optical isolator positioned along a beam path between the target
material and amplifier. In a particular embodiment, the light beam
within the amplifier may have a primary polarization direction and
the optical isolator may comprise a phase retarding optic, e.g.,
phase retarding mirror and a polarizer.
In another aspect of an embodiment of the present application, an
EUV light source may comprise a plurality of targets and a system
generating laser pulses, the laser pulses for irradiating targets
to produce a burst of EUV light pulses. For this aspect, the EUV
light source may further comprise an instrument measuring an
intensity of at least a portion of one EUV light pulse within a
burst of EUV light pulses and providing a feedback signal
indicative thereof, and a shutter responsive to the feedback signal
to trim a laser pulse, the trimmed laser pulse generating an EUV
light pulse during the burst of EUV light pulses. For this aspect,
the portion of the EUV pulse that is measured may be a temporal
portion, a spatial portion, or a temporal and spatial portion.
In one implementation of this aspect, the portion of the EUV pulse
that is measured may be a temporal portion and the EUV pulse
generating the measured intensity may be produced by the trimmed
laser pulse. For example, the system may comprise an optical
oscillator outputting laser pulses. Each output laser pulse may
have a pulse duration, t, and the travel time, T, for light
traversing an optical path extending from the optical oscillator
output through the target and to an intensity measurement site may
be less than the pulse duration exiting the oscillator, T<t. In
a particular embodiment, each output laser pulse may have a pulse
duration, t>T+T.sub.s, where T.sub.s may define a system time
including, for example, a detector response time, a processor
response time, a driver response time, a switch response time and a
signal propagation time, in one arrangement, the detector, switch
and processor may be positioned closely together to minimize
T.sub.s. Furthermore, the system may comprise at least one optical
amplifier receiving laser light from the optical oscillator along
an optical path and the shutter may be positioned on the optical
path between the optical amplifier and the optical oscillator.
In another implementation of this aspect, the EUV pulse measured to
produce the feedback signal may be generated by a corresponding
laser pulse and the feedback signal produced therefrom may be used
to trim a subsequent laser pulse. In either of these
implementations, the shutter may comprise an electro-optic switch.
For example, the system generating laser pulses may comprise an
optical oscillator defining a primary polarization direction for
light exiting the oscillator and the shutter may comprise an
electro-optic switch having an electro-optic cell, e.g. Pockels or
Kerr cell, and a polarizer.
In one implementation, the system generating laser pulses may
comprise an optical oscillator and at least one optical amplifier
and in a first particular implementation the optical oscillator may
be a cavity dumped oscillator and in a second particular
implementation the optical oscillator may be a transverse
electrical-discharge oscillator having a pulsed
electrical-discharge power source. For any of these
implementations, the amplifier may comprise a continuously pumped
laser device.
One implementation of the EUV light source employs a chamber in
which the laser pulses irradiate the targets. For this
implementation, the intensity measuring instrument may be
positioned inside the chamber, outside the chamber or in some
cases, multiple instruments may be employed, e.g. one inside and
one outside the chamber. For example, an intensity measuring
instrument may be positioned in a lithography scanner that is
coupled to the chamber and/or an intensity measuring instrument may
be positioned at a location optically downstream of a lithography
scanner that is coupled to the chamber.
In another aspect of an embodiment of the present application, an
EUV light source may comprise a plurality of targets and a system
generating laser pulses which comprises a transverse,
electrical-discharge oscillator having a pulsed
electrical-discharge power source. The laser pulses may be provided
for irradiating targets to produce a burst of EUV light pulses and
the EUV light source may further comprise an instrument measuring
an intensity of at least a portion of one EUV light pulse within a
burst of EUV light pulses, with the instrument providing a feedback
signal indicative thereof. A controller responsive to the feedback
signal may be provided to establish a discharge voltage for a
subsequent laser pulse, the subsequent laser pulse generating an
EUV light pulse during the burst of EUV light pulses. In one
particular implementation, the transverse electrical-discharge
oscillator may comprise a CO.sub.2 oscillator.
In still another aspect of an embodiment of the present
application, an EUV light source may comprise a target material and
a system having at least one optic establishing a beam path with
the target material. For this aspect, the system may provide an
initial light pulse for an initial photon interaction with the
target material, the initial interaction generating a reflected
pulse passing back and forth along the beam path between the optic
and target material and producing a plurality of subsequent photon
interactions with the target material. Each of the subsequent
interactions may produce an EUV emission and the system may have an
optical gain medium positioned along the beam path. The EUV light
source may further comprise an instrument measuring an EUV emission
intensity and providing a feedback signal indicative thereof; and a
shutter responsive to the feedback signal to selectively limit the
number of subsequent photon interactions generated by the initial
light pulse.
In a particular embodiment of this aspect, the system may comprise
an optical oscillator coupled with the beam path to provide the
initial light pulse. In one implementation the instrument may
measure an EUV emission intensity from a subsequent interaction
corresponding to an initial pulse and generate a feedback signal
for limiting the number of subsequent photon interactions generated
by the same initial light pulse. In another implementation the
instrument may measure an EUV emission intensity from a subsequent
interaction corresponding to an initial pulse and generate a
feedback signal for limiting the number of subsequent photon
interactions generated by a different, e.g. later, initial light
pulse.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic view of an overall broad conception for a
laser-produced plasma EUV light source according to an aspect of an
embodiment;
FIG. 2A shows a schematic view of portions of an EUV light source
illustrating the control EUV output intensity by varying one or
more pre-pulse parameters;
FIG. 2B shows another embodiment in which the source producing a
train of pre-pulses and the source producing a train of main pulses
may share a common pulse amplifier;
FIG. 2C shows another embodiment in which the pre-pulses and main
pulses may travel along different beam paths to reach the target
volume;
FIG. 2D shows another arrangement which may be employed to control
EUV output dose and/or pulse to pulse stability;
FIG. 2E1 shows another arrangement which may be employed to control
EUV output dose and/or pulse to pulse stability;
FIG. 2E2 shows another arrangement which may be employed to control
EUV output dose and/or pulse to pulse stability;
FIG. 2F shows a plot of EUV output intensity versus time having a
first EUV pulse corresponding to an initial LPP interaction and
subsequent EUV pulses corresponding to reflected LPP pulses;
FIG. 2G shows a plot of EUV output intensity versus time as in FIG.
2F but after a shutter has eliminated some of the "subsequent EUV
pulses" to control dose and/or pulse to pulse stability;
FIG. 3A shows a schematic view of a plurality of target material
droplets that are traveling along a common path immediately prior
to irradiation by a pre-pulse;
FIG. 3B shows a schematic view of a plurality of target material
droplets that are traveling along a common path immediately
following irradiation of a droplet by a pre-pulse;
FIG. 4 shows a schematic view of an embodiment in which the
pre-pulse and main pulse travel along parallel beam paths and are
focused using a common optic to focal spots at different
locations;
FIG. 4A shows a schematic view illustrating the focusing of a
pre-pulse for the arrangement shown in FIG. 4; and
FIG. 4B shows a schematic view illustrating the focusing of a main
pulse for the arrangement shown in FIG. 4.
DETAILED DESCRIPTION
With initial reference to FIG. 1 there is shown a schematic view of
an exemplary EUV light source, e.g., a laser produced plasma EUV
light source 20 according to one aspect of an embodiment. As shown
in FIG. 1, and described in further detail below, the LPP light
source 20 may include a system 22 for generating light pulses and
delivering the light pulses into a chamber 26. As detailed below,
the light pulses may travel along one or more beam paths from the
system 22 and into the chamber 26 to illuminate one or more targets
at an irradiation region 28.
As further shown in FIG. 1, the EUV light source 20 may also
include a target material delivery system 24, e.g., delivering
droplets of a target material into the interior of a chamber 26 to
the irradiation region 28 where the droplets will interact with one
or more light pulses, e.g., one or more pre-pulses and thereafter
one or more main pulses, to ultimately produce a plasma and
generate an EUV emission. The target material may include, but is
not necessarily limited to, a material that includes tin, lithium,
xenon or combinations thereof. The EUV emitting element, e.g., tin,
lithium, xenon, etc., may be in the form of liquid droplets and/or
solid particles contained within liquid droplets or any other form
which delivers the EUV emitting element to the target volume in
discrete amounts. For example, the element tin may be used as pure
tin, as a tin compound, e.g., SnBr.sub.4, SnBr.sub.2, SnH.sub.4, as
a tin alloy, e.g., tin-gallium alloys, tin-indium alloys,
tin-indium-gallium alloys, or a combination thereof. Depending on
the material used, the target material may be presented to the
irradiation region 28 at various temperatures including room
temperature or near room temperature (e.g., tin alloys, SnBr.sub.4)
at an elevated temperature, (e.g., pure tin) or at temperatures
below room temperature, (e.g., SnH.sub.4), and in some cases, can
be relatively volatile, e.g., SnBr.sub.4. More details concerning
the use of these materials in an LPP EUV source is provided in U.S.
patent application Ser. No. 11/406,216 filed on Apr. 17, 2006, now
U.S. Pat. No. 7,465,946, issued on Dec. 16, 2008, entitled
ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, the contents of which has
been previously incorporated by reference herein.
Continuing with FIG. 1, the EUV light source 20 may also include a
collector 30, e.g., a reflector, e.g., in the form of a truncated
ellipsoid, e.g., a graded multi-layer mirror having alternating
layers of Molybdenum and Silicon, with an aperture to allow the
light pulses generated by the system 22 to pass through and reach
the irradiation region 28. The collector 30 may be, e.g., an
ellipsoidal mirror that has a first focus within or near the
irradiation region 28 and a second focus at a so-called
intermediate point 40 (also called the intermediate focus 40) where
the EUV light may be output from the EUV light source 20 and input
to a device utilizing EUV light, e.g., an integrated circuit
lithography tool (not shown).
The EUV light source 20 may also include an EUV controller 60,
which may also include a firing control system 65 for triggering
one or more lamps and/or laser devices in the system 22 to thereby
generate light pulses for delivery into the chamber 26. The EUV
light source 20 may also include a droplet position detection
system which may include one or more droplet imagers 70 that
provide an output indicative of the position of one or more
droplets, e.g., relative to the irradiation region 28. The
imager(s) 70 may provide this output to a droplet position
detection feedback system 62, which can, e.g., compute a droplet
position and trajectory, from which a droplet error can be
computed, e.g., on a droplet by droplet basis or on average. The
droplet error may then be provided as an input to the controller
60, which can, for example, provide a position, direction and/or
timing correction signal to the system 22 to control a source
timing circuit and/or to control a beam position and shaping
system, e.g., to change the location and/or focal power of the
light pulses being delivered to the irradiation region 28 in the
chamber 26.
As further shown in FIG. 1, the EUV light source 20 may include a
droplet delivery control system 90, operable in response to a
signal (which in some implementations may include the droplet error
described above, or some quantity derived therefrom) from the
controller 60, to e.g., modify the release point of the target
material from a droplet delivery mechanism 92 to correct for errors
in the droplets arriving at the desired irradiation region 28.
For the EUV light source 20, the droplet delivery mechanism 92 may
include, for example, a droplet dispenser creating either 1) one or
more streams of droplets exiting the dispenser or 2) one or more
continuous streams which exit the dispenser and subsequently break
into droplets due to surface tension. In either case, droplets may
be generated and delivered to the irradiation region 28 such that
one or more droplets may simultaneously reside in the irradiation
region 28 allowing one or more droplets to be simultaneously
irradiated by an initial pulse, e.g., pre-pulse to form an expanded
target suitable for exposure to one or more subsequent laser
pulse(s), e.g., main pulse(s), to generate an EUV emission. In one
embodiment, a multi-orifice dispenser may be used to create a
"showerhead-type" effect. In general, for the EUV light source 20,
the droplet dispenser may be modulating or non-modulating and may
include one or several orifice(s) through which target material is
passed to create one or more droplet streams. More details
regarding the dispensers described above and their relative
advantages may be found in U.S. patent application Ser. No.
11/358,988, filed on Feb. 21, 2006, and published on Nov. 16, 2006,
as U.S. 2006/0255298A1, entitled LASER PRODUCED PLASMA EUV LIGHT
SOURCE WITH PRE-PULSE, U.S. patent application Ser. No. 11/067,124
filed on Feb. 25, 2005, now U.S. Pat. No. 7,405,416, issued on Jul.
29, 2008, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE
TARGET DELIVERY, and U.S. patent application Ser. No. 11/174,443
filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056, issued on May
13, 2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY
SYSTEM, the contents of each of which were previously incorporated
by reference.
FIG. 1 also shows, schematically, that the EUV light source 20 may
include one or more EUV metrology instruments 94 for measuring
various properties of the EUV light generated by the source 20.
These properties may include, for example, intensity (e.g., total
intensity or intensity within a particular spectral band), spectral
bandwidth, polarization, etc. For the EUV light source 20, the
instrument(s) 94 may be configured to operate while the downstream
tool, e.g., photolithography scanner, is on-line, e.g., by sampling
a portion of the EUV output, e.g., using a pickoff mirror or
sampling "uncollected" EUV light, and/or may operate while the
downstream tool, e.g., photolithography scanner, is off-line, for
example, by measuring the entire EUV output of the EUV light source
20.
FIG. 2A shows in more detail an aspect of an embodiment in which
EUV output intensity may be controlled within a burst of pulses,
and in some cases on a pulse-to-pulse basis, to provide a
predetermined energy dose, e.g., energy dose within a predetermined
range. As shown there, the system 22' may include two separate
devices 300, 302 that are used to generate the pre-pulse(s) and
main pulse(s), respectively. FIG. 2A also shows that a beam
combiner 306 may be employed to combine pulses from the devices
300, 302 along a common beam path 308. Device 300 may be a lamp,
e.g., producing incoherent light, or a laser and typically is
selected to produce output pulses having controllable energy within
a burst, and in some cases on a pulse-to-pulse basis. Suitable
lasers for use as the device 300 may include pulsed gas discharge
lasers such as excimer, CO.sub.2, etc, pulsed solid state lasers,
e.g., disk shaped Nd:YAG, etc.
Light device 302 is typically a laser and may be a different type
of laser than used for device 300. Suitable lasers for use as the
device 302 may include a pulsed laser device, e.g., a pulsed gas
discharge CO.sub.2 laser device producing radiation at 9.3 .mu.m or
10.6 .mu.m, e.g., with DC or RF excitation, operating at relatively
high power, e.g., 10 kW and high pulse repetition rate, e.g., 24
kHz or more. In one particular implementation, a continuously
pumped CO.sub.2 laser device may be used for the device 302. For
example, a suitable CO.sub.2 laser device having an oscillator and
three amplifiers (O-PA1-PA2-PA3 configuration) is disclosed in U.S.
patent application Ser. No. 11/174,299, filed on Jun. 29, 2005, now
U.S. Pat. No. 7,439,530, issued on Oct. 21, 2008, and entitled, LPP
EUV LIGHT SOURCE DRIVE LASER SYSTEM, the entire contents of which
have been previously incorporated by reference herein. Depending on
the application, other types of lasers may also be suitable, e.g.,
an excimer or molecular fluorine laser operating at high power and
high pulse repetition rate. Examples include, a solid state laser,
e.g., having a fiber or disk shaped active media, a MOPA configured
excimer laser system, e.g., as shown in U.S. Pat. Nos. 6,625,191,
6,549,551, and 6,567,450, an excimer laser having one or more
chambers, e.g., an oscillator chamber and one or more amplifying
chambers (with the amplifying chambers in parallel or in series), a
master oscillator/power oscillator (MOPO) arrangement, a power
oscillator/power amplifier (POPA) arrangement, or a solid state
laser that seeds one or more excimer or molecular fluorine
amplifier or oscillator chambers, may be suitable. Other designs
are possible.
As further illustrated by FIG. 2A, the device 300 generates a train
of pre-pulses 310a,b that are delivered to an irradiation region
28a. At the target volume, each pre-pulse may irradiate at least
one droplet(s) to produce an expanded target. Also, device 302
generates a train of main pulses 312a,b with each main pulse for
irradiating a respective expanded target at or near the irradiation
region 28a to produce an EUV light output which can then be
measured by the instrument 94 (shown in FIG. 1).
FIG. 2A also illustrates an example of a system architecture for
varying a pre-pulse parameter during the train of pre-pulses in
response to a signal from the measurement instrument 94, e.g.,
intensity detector. As shown in FIG. 2A, the controller 60 may
receive a signal, e.g., a feedback signal, e.g., a signal
indicative of EUV output intensity, from the instrument 94 and in
turn, communicate with firing control system 65 to independently
trigger one or both of the sources 300, 302 and/or control the
discharge voltage of the device 300. Although the controller 60 and
firing control system 65 are shown as discrete elements, it is to
be appreciated that the controller 60 and firing control system 65
may be integrated into a common unit and/or may share one or more
processors, memory boards, etc.
With the above-described arrangement, one or more pre-pulse
parameters may be varied such that the EUV output intensity of a
subsequent, e.g., next, EUV output pulse may be controlled. For
example, the pre-pulse parameter may be a delay time, .DELTA.t,
between a pre-pulse 310a and a corresponding main pulse 312a. This
may be accomplished by controlling the triggering times for the
devices 300, 302. Alternatively, or in addition thereto, the pulse
energy of the pre-pulse may be varied to control the EUV output
intensity. In one implementation, a processor in the controller 60
may have access to memory holding a dataset, e.g. electronic lookup
table, (which may include empirically derived and/or calculated
data) having a tabulated correspondence between one or more
pre-pulse parameters and EUV output intensity. Thus, in one
implementation, an algorithm may be processed to track a running
average of the EUV output intensity during a burst. Based on the
running average or some similar parameter, the algorithm may
further be processed to calculate a desired EUV output intensity
for a subsequent, e.g., next, EUV output pulse, selected to obtain
a desired dose. The algorithm may then query the dataset to
retrieve one or more pre-pulse parameters corresponding to the
desired EUV output intensity and initiate the transmission of a
signal indicative of the pre-pulse parameter(s) to the firing
control system 65. The firing control system 65 may then implement
the pre-pulse parameter(s) by controlling the trigger times and/or
discharge voltages for the device 300 such that a subsequent, e.g.,
next, pre-pulse/main pulse droplet irradiation is performed with
the selected pre-pulse parameter.
FIG. 2B shows another embodiment having a system 22'' in which the
source producing a train of pre-pulses and the source producing a
train of main pulses may share a common pulse amplifier 314. For
this setup, the main pulses may be generated using a four chamber
O-PA1-PA2-PA3 architecture as shown and designated 316
(oscillator), 318 (PA1), 320 (PA2), 314 (PA3). Pulses generated by
the laser device 300' may pass through the PA 314 for
amplification, prior to delivery to the irradiation region 28b, as
shown. Although three amplifiers are shown in FIG. 2B, it is to be
appreciated that more than three and as few as one amplifier may be
specified for the system 22'', For the arrangement shown in FIG.
2B, the controller 60' may receive a signal, e.g., a feedback
signal, e.g., a signal indicative of EUV output intensity, from the
measuring instrument 94' and in turn, communicate with firing
control system 65' to independently trigger the device 300' and/or
the oscillator 316 and/or control the discharge voltage of the
device 300'. In this manner, a pre-pulse parameter, e.g., a delay
time between a pre-pulse and a corresponding main pulse and/or the
pulse energy of the pre-pulse, may be varied to control the EUV
output intensity, e.g., on a pulse-to-pulse basis, and thereby
control EUV output dose and/or pulse stability.
FIG. 2C illustrates that pulses from the light sources 300'', 302''
may travel along different beam paths 322, 324 to reach the
irradiation region 28c. For the arrangement shown in FIG. 2C,
controller 60'' may receive a signal, e.g., a feedback signal,
e.g., a signal indicative of EUV output intensity, from the
measurement instrument 94'' and in turn, communicate with firing
control system 65'' to independently trigger one or more of the
devices 300'', 302'' and/or control the discharge voltage of the
device 300''. In this manner, a pre-pulse parameter, e.g., a delay
time between a pre-pulse and a corresponding main pulse and/or the
pulse energy of the pre-pulse, may be varied to control the EUV
output intensity, e.g., on a pulse-to-pulse basis, and thereby
control EUV output dose and/or pulse stability.
FIG. 2D shows another arrangement which may be employed to control
EUV output dose and/or pulse stability. As shown there, the system
22''' may include a device 326, e.g. oscillator generating laser
pulses, e.g., main pulses, with each pulse having a pulse duration,
and an amplifier 327 having one or more amplifying chambers. Note:
for some embodiments the device 326 may also provide pre-pulses or
a separate device (not shown) may be included to provide
pre-pulses, As shown, the system 22''' may also include a shutter
328 operable to alter, e.g., trim, a pulse such that only a
temporal portion of the pulse is delivered to the irradiation
region 28d to illuminate the target material.
FIG. 2D also illustrates that controller 60''' may receive a
signal, e.g., a feedback signal, e.g., a signal indicative of EUV
output intensity, from the measuring instrument 94''' and in turn,
communicate with the shutter 328. Also shown, the controller 60'''
may communicate with the firing control system 65''' to trigger
and/or control the discharge voltage of the device 326. For the EUV
light source shown in FIG. 2D, the shutter 328 (shown
schematically) may include an electro-optic switch, e.g. having a
time response in the nanosecond range, e.g., Pockel's or Kerr cell,
and a polarizer. For example, the device 326, e.g., CO.sub.2 laser
device, may employ polarizer(s) and/or Brewster's windows such that
light exiting the device 326 has a primary polarization direction.
With this arrangement, the shutter may include an electro-optic
switch and a polarizer having a transmission axis aligned
orthogonal to the primary polarization direction defined by the
device 326. Thus, when the switch is energized, light is able to
pass from the device 326 to the irradiation region 28d. On the
other hand, when the switch is de-energized, light exiting the
device 326 is rotated and is absorbed and/or reflected (away from
the beam path leading to the irradiation region 28d) by the
polarizer.
In one setup, the device 326 may be configured to produce and
output pulses having a pulse duration, t, that is longer than the
travel time, T, for light traversing the optical path extending
from the output of the device 326, through the target, and to the
intensity measurement site. With this arrangement, EUV light
generated by the leading portion of the laser pulse may be measured
and the measurement used to trim the trailing portion of the same
laser pulse. In some cases, a pulse stretcher (not shown) may be
used to provide pulses having t>T. In a particular embodiment,
each output laser pulse may have a pulse duration, t>T+T.sub.s,
where T.sub.s may define a system time including, for example, a
detector response time, a processor response time, a driver
response time, a switch response time and a signal propagation
time. In one arrangement, the measuring instrument 94'', shutter
328 and controller 60''' may be positioned closely together to
minimize T.sub.s. It is to be appreciated that this same technique
(i.e. measuring EUV produced by a leading portion of a laser pulse
and trimming the trailing portion of the same laser pulse) may also
be used on pulses whose pulse duration, t, is shorter than the
travel time, T, for light traversing the optical path extending
from the output of the device 326, through the target, and to the
intensity measurement site. For example, this functionality may be
obtained by moving the shutter 328 to a position on the optical
path that is relatively close to the irradiation site. However, the
arrangement shown in FIG. 2D in which the shutter 328 is positioned
upstream of the amplifier may be beneficial in some applications
due to the relatively low laser light intensity at the shutter
328.
The arrangement shown in FIG. 2D may also be configured to control
EUV output dose and/or pulse stability by measuring an EUV light
pulse parameter, e.g., intensity, for EUV light produced by a first
laser pulse and using the measurement to thereafter trim a
subsequent laser pulse. For this arrangement, the device 326 may
be, but is not necessarily limited to, a cavity dumped oscillator
or a transverse electric-discharge oscillator.
FIG. 2E1 shows an EUV light source having an arrangement which may
be employed to control EUV output dose and/or pulse to pulse
stability. As shown there, the light source may include a system
having a device 326', e.g., oscillator generating laser pulses,
e.g., main and/or pre-pulses, e.g., a CO.sub.2 master oscillator,
and an amplifier having one or more amplifying chambers 327a, 327b,
327c. Note: for some embodiments, a separate device (not shown) may
be included to provide pre-pulses. For the EUV light source, the
device 326' may provide an initial light pulse which is directed by
beam combiner 330 into chamber 26' where the light pulse produces
an initial photon interaction with the target material 332, e.g.
droplet produced by a droplet generator 334.
FIG. 2E1 further shows that the system may include on optic 336 and
a pair of turning mirrors 338a,b which together with the target
material 332 establish a beam path 340. For the system shown, the
optic 336 may be, for example, a reflecting mirror or corner
reflector. The EUV light source may also include a shutter 328'
switchable between a first state allowing light to travel between
the amplifier 327c and optic 336 and a second state preventing
light travel between the amplifier 327c and optic 336. For the EUV
light source, the shutter 328' may include an electro-optic switch,
e.g., having a time response in the nanosecond range, e.g.,
Pockel's or Kerr cell, and a polarizer. For example, one or more of
the amplifiers 327a-c and/or device 326', may employ polarizer(s)
and/or Brewster's windows such that light traveling between turning
mirror 338a and shutter 328' has a primary polarization direction.
With this arrangement, the shutter 328' may include an
electro-optic switch and a polarizer having a transmission axis
aligned orthogonal to the primary polarization direction defined by
the device 326'. Thus, when the switch is energized, light is able
to pass between the amplifier 327c and optic 336. On the other
hand, when the switch is de-energized, the light will have a
polarization orthogonal to the polarizer transmission axis and will
be reflected away from the beam path leading to the target material
332 by the polarizer. Alternatively, the shutter 328' may include
an acousto-optic switch, e.g., an acousto optic cell with RF
amplitude control, that is switchable between a first state
allowing light to travel between the amplifier 327c and optic 336
and a second state preventing light travel between the amplifier
327c and optic 336.
FIG. 2E1 further shows that the EUV light source may also include a
controller 60a having a processor and one or more drivers. As
shown, the controller 60a may receive a signal, e.g., a feedback
signal, e.g., a signal indicative of an EUV output intensity, from
a measuring instrument 94a and in turn, communicate with the
shutter 328'.
FIG. 2E2 shows an EUV light source arrangement (having one or more
components in common with the arrangement shown in FIG. 2E1) which
may be employed to control EUV output dose and/or pulse to pulse
stability. As shown, an EUV light source may include a device
326'', e.g., oscillator generating laser pulses (main pulses and/or
pre-pulses as described above) an amplifier having one or more
amplifying chambers 327a', 327b', 327c' (as described above) a pair
of turning mirrors 338a',b' which together with the target material
332' and at least one optic in the device 326' (e.g. output coupler
and/or rear reflector) establish a beam path 340'. For the EUV
light source, the device 326'' may provide an initial light pulse
which is directed through the amplifying chambers 327a', 327b',
327c' (which may or may not amplify the intial pulse) into chamber
26'' where the light pulse produces an initial photon interaction
with the target material 332', e.g., droplet produced by a droplet
generator 334'.
FIG. 2E2 further shows that the EUV light source may include a
shutter 328'' (e.g., electro-optic or acouto-optic as described
above) switchable between a first state allowing light to travel
between the amplifier 327c' and device 326' and a second state
preventing light travel between the amplifier 327c' and device
326'. FIG. 2E2 also shows that the EUV light source may include a
controller 60a' having a processor and one or more drivers. As
shown, the system controller 60a' may receive a signal, e.g., a
feedback signal, e.g., a signal indicative of an EUV output
intensity, from a measuring instrument 94a' and in turn,
communicate with the shutter 328''. In the operation of the EUV
light source shown in FIG. 2E2, the device 326'' may provide an
initial light pulse, e.g., laser pulse, which is directed along
beam path 340' into chamber 26'' where the light pulse produces an
initial photon interaction with the target material 332'. This
light pulse may be of sufficient intensity to produce a substantial
EUV emission, (e.g., main pulse) or may constitute a pre-pulse, as
described above. In either case, the initial interaction may
generate a reflected pulse passing back and forth along beam path
340' between one or more optics in the device 326'' and the target
material 332' producing a plurality of subsequent photon
interactions with the target material 332'. As described above, the
number of subsequent interactions may be controlled by the shutter
328'' to control dose and/or pulse set to pulse set stability.
The operation of the EUV light source shown in FIGS. 2E1 and 2E2
can best be appreciated with cross reference to FIGS. 2F and 2G. As
indicated above, the device 326', 326'' may provide an initial
light pulse, e.g., laser pulse, produces an initial photon
interaction with the target material 332, 332'. This light pulse
may be of sufficient intensity to produce a substantial EUV
emission, (e.g., main pulse) or may constitute a pre-pulse, as
described above. In either case, the initial interaction may
generate a reflected pulse passing back and forth along beam path
340, 340' between the optic 336 (FIG. 2E1) or an optic in the
device 326'' (FIG. 2E2) and target material 332 and produce a
plurality of subsequent photon interactions with the target
material 332. This is illustrated in FIG. 2F which shows an EUV
output pulse set 342 having a first EUV pulse 342a corresponding to
the initial interaction (from a main pulse) and a plurality of EUV
pulses 342b,c,d,e,f corresponding to the subsequent photon
interactions which result from the reflected pulse traveling back
and forth along the beam path 340, 340'. Significant EUV output
pulses due to the subsequent photon interactions may, at some
point, terminate, as shown, e.g., as losses exceed gain along the
beam path 340, 340'. The period between pulses in a pulse set will
depend on the length of the beam path 340, 340', and by way of
example, a beam path with an optical path length of about 56 meters
between target material 332 and optic 336 will result in a period
between peaks of about 340 ns. FIG. 2F shows that the pulse set 342
will be followed by another EUV output pulse set 344 having an EUV
output pulse 344a initiated when a "new" light pulse from the
device 326', 326'' interacts with a "new" droplet of target
material. As shown, the EUV output pulse set 344 may also include a
plurality of EUV pulses 344b,c,d,e,f corresponding to the
subsequent photon interactions which result from the reflected
pulse traveling back and forth along the beam path 340, 340'. In
general, the time period between pulse 342a and 344a is dependent
on the pulse repetition rate of the device 326, and may be, for
example, about 13.9 uS for a pulse repetition rate of about 72
kHz.
For the EUV pulse sets 342, 344 shown in FIG. 2F and described
above, correspond to an operation of the EUV light source shown in
FIGS. 2E1, 2E2 in which the shutter 328', 328'', remains open.
Thus, the number of interactions due to reflections has not been
limited by the shutter 328', 328''. For comparison, FIG. 2G shows
an EUV output pulse set 346 resulting from the operation of the EUV
light source shown in FIGS. 2E1, 2E2 in a manner such that the
number of interactions due to reflections have been selectively
limited by the shutter 328', 328'' to control dose and/or pulse set
to pulse set stability. In particular, the pulse set 346 having
pulses 346a,b,c,d has been limited to three interactions due to
reflections (pulses 346b,c,d) and the pulse set 348 having pulses
348a,b,c,d,e has been limited to four interactions due to
reflections (pulses 348b,c,d,e).
In more detail, EUV intensities corresponding to one or more of the
pulses 346a,b,c,d may be measured by instrument 94a, 94a' and used
to generate a signal, e.g. feedback signal which is communicated to
controller 60a, 60a'. The controller 60a, 60a' may, in response to
the received signal, determine the number of interactions due to
reflection required to meet a dose and/or pulse set to pulse set
stability specification. For this determination, the controller
may, in some cases, use intensity data from previous pulse sets.
After processing, the controller 60a, 60a' may then signal the
shutter 328', 328'' using an appropriate driver to de-energize the
shutter 328', 328'' at the appropriate time to limit further
interactions due to reflection. As described above, the controller
60a, 60a' may use measurements within a pulse set to limit
interactions due to reflection in that same pulse set. In one
implementation, the controller 60a, 60a' may integrate the
intensity generating a pulse energy for each pulse and then
accumulate the pulse energy for a given shot until the accumulated
pulse energy reaches a pre-selected level. At this point, the
controller 60a, 60a' may de-energize the shutter and stop
additional generation of the EUV by subsequent reflections.
For the arrangement shown in FIGS. 2E1, 2E2, the shutter 328',
328'' and controller 60a, 60a' may be located in the vicinity of
the instrument 94a, 94a' to minimize the signal transfer time.
Typically, for the case where an instrument measurement within a
pulse set is used to limit the next interaction, the EUV light
source may be configured such that the system response time (which
is equal to the sum of time of signal transfer, data integration
and analysis and optical switch delay time) is less than the
temporal separation between the reflected pulses, which, as
described above, may be about 340 ns. In an alternative
implementation, the controller 60a, 60a' may use measurements
within a pulse set to limit interactions due to reflection in a
different, e.g., subsequent, pulse set. FIG. 3A illustrates a
plurality of target material droplets that are traveling along a
common path 350, which for the illustration is a substantially
linear path. The stream may, for example be created by a droplet
dispenser as described above and be falling under the influence of
gravity. Droplet paths that are curved may also be used, for
example, the droplets may be charged and then deflected, the
droplets may follow a curve trajectory under the influence of
gravity, etc. FIG. 3A illustrates that some of the droplets on the
common path, e.g., droplet 352a may be irradiated by an initial
pulse 354, e.g., pre-pulse that is focused to a focal spot at or
near the common path 350. FIG. 3A also shows that some droplets,
e.g., droplets 352b,c may be allowed to pass through the
irradiation zone, and thus, do not participate in the production of
plasma and EUV light. Instead, for the arrangement shown in FIG.
3A, only every third droplet is irradiated with the
non-participating droplets (so-called buffer droplets) shielding
the next participating droplet from the plasma.
FIG. 3B shows the droplets after a short period of time, e.g.,
1-100 .mu.s has elapsed after the droplet 352a shown in FIG. 3A is
irradiated by an initial pulse, e.g., pre-pulse. As shown, the
effect of the initial pulse is to spatially expand the droplet
creating an expanded volume 356 that, due to the initial pulse
irradiation, has exited the path 350. FIGS. 3A and 3B further show
that a subsequent pulse 358, e.g., main pulse, may be delivered to
a location 360 that is distanced from the path 350 to expose the
expanded volume 356 and produce an EUV output. In particular, as
shown, the subsequent pulse 358 may be focused to a focal spot at
or near the location 360 such that a substantial intensity is not
presented at locations along the linear path 350. With this
arrangement, reflections to the device producing the subsequent
pulse from the buffer droplets are not created, and as a
consequence, the number of reflected photons available to cause
self lasing in the device are reduced. For the arrangement shown in
FIG. 3B, a substantial intensity is also avoided at locations along
the linear path 350 during the delivery of a subsequent pulse,
e.g., main pulse. As shown, this may be accomplished by directing
the initial pulse and subsequent pulse along different
beam-paths.
As illustrated in FIGS. 3A and 3B, the initial pulse 354 and
subsequent pulse 358 may be focused to respective focal spots in
the droplet 352a and in the expanded volume 356. However, it is to
be appreciated that the main pulse focal spot need not necessarily
lie within the target volume 356 and the pre-pulse focal spot need
not necessarily lie within the droplet 352a. Stated another way,
the initial pulse traveling along the beam path shown may be
unfocused, or alternatively, may be focused to a focal spot ahead
of or behind the droplet 352a. Similarly, the subsequent pulse
traveling along the beam path shown may be unfocused, or
alternatively, may be focused to a focal spot ahead of or behind
the expanded volume 356.
FIG. 4 shows an embodiment in which the initial pulse, e.g.,
pre-pulse and the subsequent pulse, e.g., main pulse may travel
along substantially parallel beam paths 400, 401 and may be focused
using a common optic 402. The spacing of the parallel beam paths
may compensate for movement of material in the expanding volume.
The focusing of an initial pulse relative to the linear path 350'
for the arrangement shown in FIG. 4 is illustrated in FIG. 4A and
the focusing of a subsequent pulse is shown in FIG. 4B. As
described below, for this embodiment, a substantial intensity may
be avoided at locations along the linear path 350' during delivery
of the subsequent pulse (reducing droplet reflections and/or laser
droplet coupling and the associated self-lasing). Stated another
way, the subsequent pulse may be in a substantially de-focused
condition at locations along the linear path 350' as illustrated in
FIG. 4B. As shown, this may be performed while still focusing the
initial pulse 354' at or near the droplet 352a' (FIG. 4A) and
focusing the subsequent pulse 358' at or near the expanded volume
356' (FIG. 4B). This may be accomplished, for example, by using an
initial pulse beam and a subsequent pulse beam that have different
beam divergences prior to focusing. This difference in divergence
may arise because the initial pulse and subsequent pulse may be
initiated by different sources 300''', 302''' and/or one or more
optical arrangements may be provided to alter the beam divergence
of the initial pulse beam and/or subsequent pulse beam.
As further shown in FIG. 4, the EUV light source 20' may include an
optical isolator 404 positioned along the beam path 400 to reduce
the number of reflected photons, e.g., photons reflected from
droplets, from reaching the light sources 300''' and or 302'''
where the photons may cause self lasing and reduce the efficiency
of the laser gain media to produce controllable, repeatable pulses.
For the light source 20', the optical isolator 404 may include one
or more saturable absorbers. For example, a chamber having opposed
windows and filled with a saturable absorber material may be
provided. The type of saturable absorber material may be selected
for the wavelength of interest, e.g., SF.sub.6 gas for light at a
wavelength of 10.6 .mu.m. Alternatively, or in addition to the
saturable absorber(s), the optical isolator 404 may include a
switchable shutter or a so-called isolator box (details not
shown).
In one construction of an isolator box, the source 302''', e.g.,
CO.sub.2 laser device may employ polarizer(s) and/or Brewster's
windows such that light exiting the source 302''' has a primary
polarization direction. With this arrangement, the isolator box may
include, for example, a phase retarder mirror which rotates back
reflected light ninety degrees out of the primary polarization
direction and an isolator mirror which absorbs light with the
rotated polarization. For example, a suitable unit for use with
CO.sub.2 lasers may be obtained from Kugler GmbH, Heiligenberger
Str. 100, 88682 Salem Germany under the trade name Queller and/or
"isolator box". Typically, the isolator box functions to allow
light to flow from the source 302''' to the droplet substantially
unimpeded while allowing only about one percent of back-reflected
light to leak through the isolator box and reach the source
302'''.
It will be understood by those skilled in the art that the aspects
of embodiments of the present invention disclosed above are
intended to be preferred embodiments only and not to limit the
disclosure of the present invention(s) in any way and particularly
not to a specific preferred embodiment alone. Many changes and
modification can be made to the disclosed aspects of embodiments of
the disclosed invention(s) that will be understood and appreciated
by those skilled in the art. The appended claims are intended in
scope and meaning to cover not only the disclosed aspects of
embodiments of the present invention(s) but also such equivalents
and other modifications and changes that would be apparent to those
skilled in the art. While the particular aspects of embodiment(s)
described and illustrated in this patent application in the detail
required to satisfy 35 U.S.C. .sctn.112 are fully capable of
attaining any above-described purposes for, problems to be solved
by or any other reasons for or objects of the aspects of an
embodiment(s) above described, it is to be understood by those
skilled in the art that it is the presently described aspects of
the described embodiment(s) of the present invention are merely
exemplary, illustrative and representative of the subject matter
which is broadly contemplated by the present invention. The scope
of the presently described and claimed aspects of embodiments fully
encompasses other embodiments which may now be or may become
obvious to those skilled in the art based on the teachings of the
Specification. The scope of the present invention is solely and
completely limited by only the appended claims and nothing beyond
the recitations of the appended claims. Reference to an element in
such claims in the singular is not intended to mean nor shall it
mean in interpreting such claim element "one and only one" unless
explicitly so stated, but rather "one or more". All structural and
functional equivalents to any of the elements of the
above-described aspects of an embodiment(s) that are known or later
come to be known to those of ordinary skill in the art are
expressly incorporated herein by reference and are intended to be
encompassed by the present claims. Any term used in the
Specification and/or in the claims and expressly given a meaning in
the Specification and/or claims in the present application shall
have that meaning, regardless of any dictionary or other commonly
used meaning for such a term. It is not intended or necessary for a
device or method discussed in the Specification as any aspect of an
embodiment to address each and every problem sought to be solved by
the aspects of embodiments disclosed in this application, for it to
be encompassed by the present claims. No element, component, or
method step in the present disclosure is intended to be dedicated
to the public regardless of whether the element, component, or
method step is explicitly recited in the claims. No claim element
in the appended claims is to be construed under the provisions of
35 U.S.C. .sctn.112, sixth paragraph, unless the element is
expressly recited using the phrase "means for" or, in the case of a
method claim, the element is recited as a "step" instead of an
"act".
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